vlib: process node scheduler rework

vlib: process node scheduler rework

This commit allow use od cooperative multitasking with multiple
descheduling reasons (i.e. event wait and suspend) inside the same
process node. In previus code remote node will wake up process node
by sending event evein if process node is waiting in
vlib_process_ssupend().

This change also allowed new vlib_process_yield() API which deschedules
current process and it puts it into the end of queue.

Change-Id: I846e5a99b4ea1809eb80895f6ffe0ef0b2fd21ae
Type: improvement
Signed-off-by: Damjan Marion <[email protected]>